unsubscribeSet (sub_set, sub_receiver)
Unsubscribe node sub_receiver from subscription set sub_set. This is
invoked on the receiver node. It verifies that this does not break
any chains (e.g. - where sub_receiver is a provider for another node),
then restores tables, drops Slony-specific keys, drops table entries
for the set, drops the subscription, and generates an UNSUBSCRIBE_SET
node to publish that the node is being dropped.